1. Field of the Invention
This application relates to electric lamps and in particular, to electric lamps having shrouds.
2. Discussion of the Art
Metal halide arc discharge lamps are frequently employed in commercial usage because of their high luminous efficacy and long life. A typical metal halide arc discharge lamp includes a quartz or ceramic arc tube that is hermetically sealed within a glass jacket or outer envelope. The arc tube, itself hermetically sealed, has tungsten electrodes frit or press sealed in opposite ends and has a bulb portion containing fill material including mercury, metal halide additives, and a rare or noble gas to facilitate starting. The outer envelope is either evacuated or filled with nitrogen or another inert gas at less than atmospheric pressure.
The metal halide arc tube is often surrounded with a shroud which comprises a generally cylindrical tube of light-transmissive material, such as quartz, that is able to withstand high operating temperatures. The arc tube and the cylindrical shroud are coaxially mounted within the lamp outer envelope with the arc tube located within the shroud. The shroud improves the safety of the lamp by acting as a containment device in the event that the arc tube shatters. The shroud allows the lamp outer envelope to remain intact by dissipating the energy of a shattering arc tube. The presence of a shroud expands the market for metal halide lamps into open-type (absence of an expensive cover plate) lighting fixtures.
Sodium is an important constituent in metal halide arc discharge lamps, usually in the form of sodium iodide. Sodium is used to improve the efficacy and color rendering properties. It has long been recognized that quartz arc tubes containing sodium lose sodium during operation by movement or migration through the arc tube wall. The iodine originally present in a metal halide arc discharge lamp as sodium iodide is freed by sodium loss, and the iodine combines with mercury in the arc tube to form mercury iodide. Mercury iodide leads to increased reignition voltages, thereby causing starting and lamp maintenance problems and shortening lamp life.
There is evidence that most of the sodium loss is due to a negative charge on the arc tube walls caused by photoelectric emission from electrified side rods used to support the arc tube and shroud within the outer envelope. Solutions to this problem are known in the art. See, for example, U.S. Pat. No. 5,493,167, where a ceramic sleeve and insulator support stops are used to prevent sodium loss. While such lamp constructions provide an improvement, the structures are complex and still require a relatively high number of parts and/or welds, making them difficult to assemble.
Accordingly, a need exists for a more efficient lamp construction.
In accordance with the present invention, the improved electric lamp includes a sealed light-transmissive outer envelope having an interior space, a base fixed to the outer envelope, a non-insulated main conductor wire within the outer envelope and connected to the base at one end, a light source capable of generating light within the outer envelope, a shroud surrounding the light source and mounted adjacent the non-insulated main conductor wire, and a first center support. The light source has first and second ends. The first end is electrically coupled to the non-insulated main conductor wire, and the second end is coupled to a stem lead. The first center support supports the shroud and the light source and mechanically couples the shroud and the light source to the non-insulated main conductor wire.